The digital memory most commonly used in computers and computer system components is the dynamic random access memory (DRAM), wherein voltage stored in capacitors represents digital bits of information. Electric power must be supplied to the capacitors to maintain the information because, without frequent refresh cycles, the stored charge dissipates, and the information is lost. Memories that require constant power are known as volatile memories.
Non-volatile memories do not need frequent refresh cycles to preserve their stored information, so they consume less power than volatile memories and can operate in an environment where the power is not always on. There are many applications where non-volatile memories are preferred or required, such as in cell phones or in control systems of automobiles. Non-volatile memories include magnetic random access memories (MRAMs), erasable programmable read only memories (EPROMs) and variations thereof.
Another type of non-volatile memory is the programmable conductor or programmable metallization memory cell, which is described by Kozicki et al. in (U.S. Pat. Nos. 5,761,115; 5,914,893; and 6,084,796) and is included by reference herein. The programmable conductor cell of Kozicki et al. (also referred to by Kozicki et al. as a “metal dendrite memory”) comprises a glass ion conductor, such as a chalcogenide-metal ion glass and a plurality of electrodes disposed at the surface of the fast ion conductor and spaced a distance apart from one another. The glass/ion element shall be referred to herein as a “glass electrolyte,” or, more generally, “cell body.”
When a voltage is applied to the anode and the cathode, a non-volatile conductive pathway (considered a sidewall “dendrite” by Kozicki et al.) grows from the cathode through or along the cell body towards the anode, shorting the electrodes and allowing current flow. The dendrite stops growing when the voltage is removed. The dendrite shrinks, re-dissolving metal ions into the cell body, when the voltage polarity is reversed. In a binary mode, the programmable conductor cell has two states; a fully-grown dendrite or shorted state that can be read as a 1, and a state wherein the dendrite does not short out the electrodes that can be read as a 0, or vice versa. It is also possible to arrange variable resistance or capacitance devices with multiple states.
The recent trends in memory arrays generally have been to form first a via, then fill it with a memory storage element (e.g., capacitor) and etch back. It is simple to isolate individual memory cells in this way. Programmable memory cells also have been fabricated using this so-called container configuration, wherein the electrodes and cell body layers are deposited into a via that has been etched into an insulating layer. Metal diffusion in the course of growing and shrinking the conductive pathway is confined by the via wall. The memory cell can be formed in a number of array designs. For example, in a cross-point circuit design, memory elements are formed between upper and lower conductive lines at intersections. When forming a programmable conductor array with the glass electrolyte elements similar to those of Kozicki et al., vias are formed in an insulating layer and filled with the memory cell bodies, such as metal-doped glass electrolyte or glass fast ion diffusion (GFID) elements.
Accordingly, a need exists for improved methods and structures for forming integrated programmable conductor memory arrays.